[format.parse.ctx] fix incorrect mention of basic_format_args

Co-authored-by: Jonathan Wakely <[email protected]>

source/utilities.tex

@@ -17421,7 +17421,7 @@
 \tcode{id < num_args_} is \tcode{true} and
 \item
 the type of the corresponding format argument
-(after conversion to \tcode{basic_format_args<Context>}) is one of the types in \tcode{Ts...}.
+(after conversion to \tcode{basic_format_arg<Context>}) is one of the types in \tcode{Ts...}.
 \end{itemize}
 \end{itemdescr}